On Wed, 2020-11-25 at 11:02 +0530, Nikhil Shetty wrote: > > On Tue, Nov 24, 2020 at 9:35 PM Laurenz Albe <laurenz.albe@xxxxxxxxxxx> wrote: > > On Tue, 2020-11-24 at 19:54 +0530, Nikhil Shetty wrote: > > > We have one Primary and three Standby Postgresql setup. Two standby in DC and one standby in DR. > > > > > > We have configured streaming replication.When there are a lot of write transactions > > > in the master, we observed that DR lags behind the master for 5-6 hours. We initially > > > thought this could be because of the slow network between DC and DR and this is normal > > > but when we saw the replay_lsn and receive_lsn , they had a huge difference. > > > > > > DR standby was receiving the WAL's in time and there was no delay, the delay was in > > > replaying the lag. Can anybody point me to where I can start with the investigation? > > > DC (Master and 2 Standby) and DR(Standby) have the same setup in terms of storage > > > and some database configuration parameters but DR has a little less CPU and RAM. > > > > If there is a delay replaying WAL, the cause is probably a replication conflict. > > > > Set "hot_standby = off" on the standby to avoid the problem totally. > > > > If you want queries on the standby, set "max_standby_streaming_delay" to 0 > > to keep replication from falling behind. > > We don't have any queries running on the DR server so setting "hot_standby=off" > can be done and we will observe the WAL replay but ss there a way to identify > that replay lag is because of the queries running on DR. I don't see any long > running queries in pg_stat_activity to > suggest that queries are not allowing the replay to happen. You could look for conflicts in "pg_stat_database" on the standby. If that is not the cause, perhaps the network is too slow for the amount of WAL, or the standby cannot replay fast enough (the startup process is a single thread). Yours, Laurenz Albe
